TRADITIONALLY IN polite society no one is supposed to talk about religion, sex, or politics. Conversation about careers, children, holidays, and pension plans is fine; middle-class people share these concerns, while religion is one of those little complications that can wreck a good dinner party.

Even for people who are religious, God is a Sunday activity; in terms of their everyday behaviour, such people do not differ very much from their non-religious friends. All of them share a common world view, it is just that the believers tag on God as the "something" which started the world, while the atheists find the idea of such a prime mover unnecessary. On this view a disagreement over God doesn't actually matter very much.

Yet turn to Augustine, Bishop of Hippo (354-430AD), one of the world's most distinguished Christians, and compare him with Nietzsche (1844-1900), one of the world's most distinguished atheists; you find both agree that belief in God affects everything. This agreement is surprising, given that they seem complete opposites. But, dig a little further, and you find a striking similarity: they both believed that the possibility of "truth" depended on belief in God. They both thought that a person's confidence that "x is true" is making all sorts of assumptions about the nature of the world; for Augustine, the fact that we make truth claims is clear evidence that God exists; for Nietzsche, our cultural sense that God is no longer an option for us means that we cannot any longer have any confidence in truth.

So why should the possibility of truth depend on God? Augustine and Nietzsche use different arguments. In On the Free Choice of the World, Augustine argues that truth must be ultimately immutable and therefore exists in a way that transcends the human mind; the unity and existence of truth are, in part, what we mean by God. Nietzsche's argument concentrates on the odd way that rationality emerged from a non- rational process like evolution; evolution is a blind, random process; given this, how can anyone trust the resulting order of rationality? What both men are getting at is that there is something extraordinary about the capacity to argue, think, and reason.

The claim that I know that there are four chairs in my study assumes a stable world; one that the human mind has the capacity to interpret accurately; a world that is ultimately intelligible and makes sense. If we decide that it is just lucky that the world is intelligible, then it is difficult to see how we can be confident it is, in fact, intelligible. And if we cannot be confident, then it is difficult to see how we are justified in saying "something is true". So we need an explanation for the intelligibility of the world. And the best explanation for our truth assumptions is a Creator God. A world which is intended is a world that makes sense.

If Augustine is right then, if there is such a thing as objective truth, atheism is an impossible position. One cannot decide that "there is no God" is a true statement. Truth depends on God so it cannot be true that there is no God. Nietzsche took a different route. He decided that there was no truth and therefore everything is projection, even science. We paint pictures from our experience of the world; but there is no way of preferring one over another; we are in a post- modern fog of projection and pictures.

So, all those atheists who, perhaps for moral or scientific reasons, find belief difficult, face a hard choice. If their conviction that science is true is justified, then they need to explain what is the basis for that conviction. Can science rest upon the shifting sands of post-modernity or can atheist scientists posit a credible alternative to Augustine's notion about the nature of absolute truth and what it is which explains the intelligibility of the world? In the end they will be forced back upon what is for them the paradox that the best reason for belief in God is our belief in reason itself.

There is one last danger. A culture dominated by atheism would be a culture in trouble. It is no coincidence that secularism has run in parallel with ethical relativism and extreme individualism. Ethical respect does depend on ethical truth. It is because it is true that it is wrong to deny freedom to others that I support the rights of others. Toleration depends on truth. If there is no reason to be committed to human freedom, then there is nothing, intrinsically, wrong with totalitarianism. It is not surprising that the major religions of the world are increasingly convinced that the language of human rights needs God.

Despite all the hysterical voices predicting the demise of belief in Britain, religion is surprisingly robust. The task now for the major world religions is to explain to the secular world why that is a good thing. We need good arguments for faith. Once one appreciates precisely what is at stake, then the arguments are overwhelming. Everything we assume about life speaks to us about God. Recognition of this is why we have occasionally to spoil those delightful dinner parties.

Ian Markham is the Liverpool Professor of Theology and Public Life at Liverpool Hope University College. He is the author of `Truth and the Reality of God' (T & T Clark, pounds 18.95)